Several types of memory devices, such as Flash memories, use arrays of analog memory cells for storing data. Each analog memory cell stores a quantity of an analog value, also referred to as a storage value, such as an electrical charge or voltage. This analog value represents the information stored in the cell. In Flash memories, for example, each analog memory cell holds a certain amount of electrical charge. The range of possible analog values is typically divided into intervals, each interval corresponding to one or more data bit values. Data is written to an analog memory cell by writing a nominal analog value that corresponds to the desired bit or bits.
Some memory devices, commonly referred to as Single-Level Cell (SLC) devices, store a single bit of information in each memory cell, i.e., each memory cell can be programmed to assume two possible programming levels. Higher-density devices, often referred to as Multi-Level Cell (MLC) devices, store two or more bits per memory cell, i.e., can be programmed to assume more than two possible programming levels.
Several techniques are known in the art for protecting data stored in analog memory cell devices against power interruptions and programming failures. For example, U.S. Patent Application Publication 2007/0143531, whose disclosure is incorporated herein by reference, describes a method for power loss recovery for bit alterable memory. A bit alterable memory device includes status bits for a colony of memory cells. The state of each status bit may be changed depending on the programming state of the non-volatile bit alterable memory. The status bits are examined to determine the write status of two separate colonies of memory cells in the event of a power loss. The information gathered from the status bits is used by a power loss recovery mechanism to determine whether the data written to a plurality of memory cell colonies is partially written.
As another example, U.S. Pat. No. 6,988,175, whose disclosure is incorporated herein by reference, describes a method for managing page-based data storage media such as Flash media. New data are written to the storage medium in a manner that precludes corruption of old data if the writing of the new data is interrupted. Specifically, risk zones are defined by identifying, for each page, the other pages whose data are put at risk of corruption if writing to the page is interrupted. A page, which otherwise would be the target of a write operation, is not written if any of the pages in its risk zone contain data that could be corrupted if the write operation is interrupted.
U.S. Pat. No. 7,420,847, whose disclosure is incorporated herein by reference, describes a non-volatile memory device that is able to recover data in the event of a program failure without having to maintain a copy of the data until the write is completed. In some embodiments, the data is recovered by logically combining the verify data for the (failed) write process maintained in data latches with the results of one or more read operations to reconstitute the data.